1. Field of the Invention
The present invention relates to a solid state image sensor using a CCD, and, more particularly, to a solid state image sensor with an improved signal detector which finally extracts signal charges from a CCD type image sensing channel.
2. Description or the Related Art
Conventionally, a solid state image sensor employs a signal detector using a floating diffusion type amplifier (FDA) or a floating well type detector (FWD) in order to detect signal charges from a CCD type image sensing channel In the FDA, a source follower type amplifier of an E/D type is connected to a signal pick-up terminal from a image sensing channel. In the FWD, transistors are formed in a direction perpendicular to the CCD charge transfer direction (Draft 2-12, Television Society National Convention in 1988). This FWD prevents noise generation by an interfacial trap because a current flowing through a p type channel, i.e., a hole current, is modulated by signal charges (electrons) transferred through an n type buried channel so as not to flow through an Si-SiO.sub.2 interface. Also, making a gate oxidized film 1 .mu.m thick can reduce the amount of a detected capacitance and provide a high sensitivity. A floating potential plate for making the potential of a channel remote from a gate electrode uniform is provided, and further, a load transistor and an E/D source follower type amplifier are provided.
However, such amplifier have been the following problems. Regarding the FDA, the amount of saturating signals is large enough, but there is relatively large noise, which corresponds to 20 to 40 electrons in terms of the number of electrons packet (refer to Television Society Magazine Vol. 39, No. 12 (1985), pp. 1176-1181). In the FWD, on the other hand, the amount of saturating signals is as small as approximately 2000 electrons although the number of noise electrons is less than one electron.
Therefore, conventionally, there has been a tradeoff relation such that a signal detector with less noise has a smaller amount of saturating signals and a signal detector with a large amount of saturating signals generates greater noise.